In the past few decades, the growing demand for energy combined with declining fossil fuel reserves has created a tremendous surge in interest for efficient manufacturing of fuels and bulk chemicals from renewable bioresources. The natural heterobiopolymer lignin has developed into a major target for cost-efficient biomass conversion because the repeating aromatic ether structural units could offer high energy content products and potential access to useful derivatives for fine chemical applications. However, at present, utilization of lignin is clearly limited since current technology does not allow for efficient decomposition into its constituent building blocks with the desired selectivity. One of the major challenges associated with such a process is the need to reductively cleave the different types of strong aromatic C—O bonds present in lignin (FIG. 1), which is also a relevant problem for the liquefaction of coal.
Additional challenges are faced in the processing of coal and petroleum products, where increasing environmental regulations require the virtual elimination of sulfur from feedstreams. Combustion of sulfur leads to sulfur oxides with are environmentally undesirable in their own rights, but also tend to poison precious metal catalysts used, for example, in catalytic converters. There is a high interest in technologies which not only depolymerize biomass, but which act to reduce or eliminate residual sulfur from these feedstock matrices.
Ni catalysts are known to provide selective reductive transformations involving aryl-oxygen bonds, but only at loadings of 5-20%, and at these levels the use of Ni and other transition-metal catalysts are problematic, both from an economic and environmental perspective. Further, such catalysts are not reported to be useful on C—N or C—S bonds. And while it would be beneficial to have a general methodology for aromatic C—O bond cleavage that does not employ nickel or other transition metal catalysts, the only known alternative approaches for metal free ether cleavage at relatively low temperatures rely on excess alkali metals or electrocatalytic processes that tend to be costly, unsustainable and impractical.
Separately, the ability to silylate organic moieties has attracted significant attention in recent years, owing to the utility of the silylated materials in their own rights, or as intermediates for other important materials used, for example, in agrichemical, pharmaceutical, and electronic material applications. Further, the ability to functionalize polynuclear aromatic compounds with organosilanes provides opportunities to take advantage of the interesting properties of these materials.
Historically, the silylation of aromatic compounds has been achieved via free radical processes involving thermally, photochemically, or by otherwise derived radical sources. More recently, the transition metal mediated aromatic C—H silylation has been described, with different systems described based on, for example, Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, and Pt catalysts. But for certain electronic applications, the presence of even low levels of such residues can adversely affect the performance of the silylated materials. Similarly, in certain pharmaceutical applications, limits on residual transition metals are fairly strict, and the ability to avoid them entirely offers benefits during post-synthesis work-up.
The present invention is directed at solving at least some of the problems in both of these areas.